The present invention relates to light emitting diodes (LEDs) and in particular relates to LEDs formed with active portions of Group III nitrides on silicon carbide substrates.
Lighting Technologies
Lighting devices for many applications fall into several broad categories. Incandescent lighting is produced by heating a metal filament, usually by passing electric current through a metal filament. The heated filament emits light. Household and other indoor lighting is a common application of incandescent lighting. “Halogen” lighting works on the same general principle, but more efficiently. Fluorescent light is generated by exciting a vapor (typically mercury-containing) with an applied potential difference. Photons emitted by the excited vapor then strike phosphors that emit the visible light. Fluorescent light is also common in household, office and a variety of other applications.
Light Emitting Diodes
A light emitting diode (LED) is a p-n junction semiconductor diode that emits photons when forward biased. Thus, light emitting diodes produce light based upon the movement of electrons in a semiconductor material. Therefore, LEDs do not require (although they can be used in conjunction with) vapors or phosphors. They share the desirable characteristics of most semiconductor-based devices, including high efficiency (their emissions include little or no heat), high reliability and long life. For example, typical LEDs have a mean time between failures of between about 100,000 and 1,000,000 hours meaning that a conservative half lifetime for an LED is on the order of 50,000 hours.
In particular, an LED's emitted light has a frequency (which in turn relates directly to wavelength and color in accordance with well-understood principles of physics) based upon the energy difference between permitted energy levels in the material, a characteristic referred to as the bandgap. The bandgap is a fundamentally property of the semiconductor material and its doping. Thus, LEDs formed in silicon (Si, bandgap of 1.12 electron volts (eV)) will have energy transitions in the infrared (but not the visible) portions of the spectrum. Silicon-based diode are thus used for items such as low-cost sensors in which visibility to the human eye is either unimportant or specifically undesired. LEDs formed in gallium arsenide (bandgap 1.42 eV), or most commonly in silicon-doped aluminum gallium arsenide (AlGaAs) will emit in the visible portion of the spectrum, but at lower frequencies that produce infrared radiation and red and yellow light.
In turn, because green, blue, and ultraviolet (UV) photons represent higher frequency colors (E=hν) within (and beyond) the visible spectrum, they can only be produced by LEDs with bandgaps of at least about 2.2 eV. Such materials include diamond (5.47 eV), silicon carbide (2.99 eV) and Group III nitrides such as GaN (3.4 eV). In addition to producing green, blue or ultraviolet light per se, wide bandgap LEDs can be combined with red and green LEDs to produce white light, or with phosphors that produce white light when excited by blue or UV light, or both.
For several reasons, the Group III nitride compositions (i.e., Group III of the periodic table), particularly GaN, AlGaN, InGaN and AlInGaN are particularly useful for blue-emitting LEDs. As one advantage, they are “direct” emitters, meaning that when an electron transition occurs across the bandgap, much of the energy is emitted as light. By comparison, “indirect” emitters (such as silicon carbide) emit their energy partially as light (a photon) and predominantly as vibrational energy (a phonon). Thus Group III nitrides offer efficiency advantages over indirect transition materials.
As another advantage, the bandgap of ternary and quaternary Group III materials (e.g., AlGaN, InGaN, AlInGaN) depends upon the atomic fraction of the included Group III elements. Thus the wavelength (color) of the emission can be tailored (within limits) by controlling the atomic fraction of each Group III element in a ternary or quaternary nitride.
Wide bandgap semiconductors have been, however, historically more difficult to produce and work with than gallium-arsenide or gallium phosphide (GaP). As a result, blue and UV-emitting LEDs have lagged behind GaP-based LED's in their commercial appearance. For example, silicon carbide is physically very hard, has no melt phase, and requires high temperatures (on the order of about 1500-2000° C.) for epitaxial or sublimation growth. The Group III nitrides have relatively large nitrogen vapor pressures at their melting temperatures and thus are likewise difficult or impossible to grow from a melt. Additionally, difficulties in obtaining p-type gallium nitride (and other Group III nitrides) remained a barrier to diode production for a number of years. Accordingly, the commercial availability of blue and white-emitting LEDs is more recent than the corresponding availability of GaP-based and GaAs-based LEDs.
Nevertheless, based on more-recent developments, blue LED's and derivative white-emitting solid state lamps based upon Group III nitrides have become increasingly common in solid state lighting applications.
Quantity of Light Output
For comparison and other relevant purposes, lighting is typically quantified as to its output. One typical unit of measure is the lumen, defined as a unit of luminous flux equal to the light emitted in a unit solid angle by a uniform point source of one candela (cd) intensity. In turn, the candela is the base unit of luminous intensity in the International System of Units that is equal to the luminous intensity in a given direction of a source which emits monochromatic radiation of frequency 540×1012 hertz and has a radiant intensity in that direction of 1/683 watt per unit solid angle.
Using lumens as the unit of measurement, an intensity of 1200-1800 lumens is typical of incandescent bulbs and 1000-6000 lumens (depending upon circumstances) is typical in natural daylight. Light emitting diodes, however, are much less intense, for example on the order of about 10-100 lumens. One reason is their small size. Thus, applications for single (or small groups of) LEDs have historically gravitated towards indication (e.g. the register of a hand-held calculator) rather than illumination (a reading lamp). Although the availability of blue LEDs and corresponding white-emitting devices have moved such LEDs into wider commercial availability, for illumination purposes, several (or more) LEDs are typically grouped together to provide the desired output.
Because of their typical size and structure, the output of LEDs is often measured in units other than lumens. Additionally, an LED's output also depends upon the applied current, which in turn depends upon the potential difference applied across the diode. Thus, the output of an LED is often referred to as its radiant flux (Rf) and is expressed in milliwatts (mW) at a standard 20 milliamp (mA) drive current.
In particular, blue LEDs and their related derivative devices are becoming more frequently included in consumer electronic devices, particularly small displays. Common examples include items such as computer screens, personal digital assistants (“PDAs”) and cellular phones. In turn, these small devices drive demand for LEDs with reduced size (“footprint”). Such LEDs, however, must still operate at low forward voltages (Vf) and high light output. To date, however, reducing the size of the Group III nitride devices has tended to increase their forward voltage and reduce their radiant flux.
Accordingly, a need exists for continual improvement in the output of small-size LEDs formed in wide bandgap materials.